linux-kernel - [PATCH] sysctl: avoid spurious permanent empty tables

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20240827-sysctl-const-shared-identity-v1-1-2714a798c4ff@weissschuh.net>
Date: Tue, 27 Aug 2024 11:54:43 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Luis Chamberlain <mcgrof@...nel.org>, Kees Cook <kees@...nel.org>, 
 Joel Granados <j.granados@...sung.com>
Cc: Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
 llvm@...ts.linux.dev, kernel test robot <oliver.sang@...el.com>, 
 stable@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH] sysctl: avoid spurious permanent empty tables

The test if a table is a permanently empty one, inspects the address of
the registered ctl_table argument.
However as sysctl_mount_point is an empty array and does not occupy and
space it can end up sharing an address with another object in memory.
If that other object itself is a "struct ctl_table" then registering
that table will fail as it's incorrectly recognized as permanently empty.

Avoid this issue by adding a dummy element to the array so that the
array is not empty anymore and the potential address sharing is avoided.
Explicitly register the table with zero elements as otherwise the dummy
element would be recognized as a sentinel element which would lead to a
runtime warning from the sysctl core.

While the issue seems unlikely to be encountered at this time, this
seems mostly be due to luck.
Also a future change, constifying sysctl_mount_point and root_table, can
reliably trigger this issue on clang 18.

Given that empty arrays are non-standard in the first place,
avoid them if possible.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202408051453.f638857e-lkp@intel.com
Fixes: 4a7b29f65094 ("sysctl: move sysctl type to ctl_table_header")
Fixes: a35dd3a786f5 ("sysctl: drop now unnecessary out-of-bounds check")
Cc: stable@...r.kernel.org
Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
This was originally part of a feature series [0], but is resubmitted on
its own to make it into v6.11To.

[0] https://lore.kernel.org/lkml/20240805-sysctl-const-api-v2-0-52c85f02ee5e@weissschuh.net/
---
 fs/proc/proc_sysctl.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 9553e77c9d31..d11ebc055ce0 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -29,8 +29,13 @@ static const struct inode_operations proc_sys_inode_operations;
 static const struct file_operations proc_sys_dir_file_operations;
 static const struct inode_operations proc_sys_dir_operations;
 
-/* Support for permanently empty directories */
-static struct ctl_table sysctl_mount_point[] = { };
+/*
+ * Support for permanently empty directories.
+ * Must be non-empty to avoid sharing an address with other tables.
+ */
+static struct ctl_table sysctl_mount_point[] = {
+	{ }
+};
 
 /**
  * register_sysctl_mount_point() - registers a sysctl mount point
@@ -42,7 +47,7 @@ static struct ctl_table sysctl_mount_point[] = { };
  */
 struct ctl_table_header *register_sysctl_mount_point(const char *path)
 {
-	return register_sysctl(path, sysctl_mount_point);
+	return register_sysctl_sz(path, sysctl_mount_point, 0);
 }
 EXPORT_SYMBOL(register_sysctl_mount_point);
 

---
base-commit: 3e9bff3bbe1355805de919f688bef4baefbfd436
change-id: 20240827-sysctl-const-shared-identity-9ab816e5fdfb

Best regards,
-- 
Thomas Weißschuh <linux@...ssschuh.net>